Assessment of corticospinal and somatosensory conduction simultaneously during scoliosis surgery.

The function of descending motor pathways and that of ascending sensory pathways in the spinal cord were monitored at the same time in 120 patients undergoing surgery for scoliosis. Transcranial electrical stimulation of the motor cortex was performed simultaneously with stimulation of the tibial nerves in the popliteal fossae, and the descending and ascending volleys were recorded from the spinal cord at two levels using epidural electrodes. Stable recordings of both volleys have been obtained in all neurologically normal patients and in many with pre-existing neurological deficits. The experimental conditions which resulted in reliable recordings were explored in select patients and include: a vertex-anode/lateral cathode montage for transcranial stimulation, epidural recording of evoked corticospinal and somatosensory volleys at two spinal levels, a high-pass filter of 500 Hz, and stable anaesthesia. The epidural recording allows full muscle relaxation and the use of volatile anaesthetics; recording at two levels allows a deterioration in function to be identified quickly and distinguished from an artifactual change.